where l is the mean free path, n is the number density
(with units of number per cubic centimeter) and σ is the
collision cross section (with units of square centimeters). One should
note that the mean free path decreases with increasing density
and/or cross section. This makes intuitive sense -- if you encounter more
particles or particles with larger sizes, chances are greater that you will
bump into one sooner, meaning that the mean distance you travel before hitting one is shorter.

The concept of mean free path is used in many fields of physics, and is
particularly important in the fields of gaseous diffusion, radiation
physics and radiative transfer of photons through gases and
plasmas, and nuclearreactions. The equation above is quite simple, but the physics of
cross sections is more complicated, particularly when dealing with charged
particles. It is also a statistical quantity since individual particles may
travel more or less than the mean free path in a given event.